Optical fiber glass preform and method for manufacturing optical fiber glass preform

ABSTRACT

Provided is an optical fiber glass preform in which a starting rod and a dummy glass are hardly separated from each other, and a method for manufacturing the glass preform. In the optical fiber glass preform, the dummy glass is fitted into one end of the starting rod, and a part of the dummy glass and the starting rod are surrounded by a clad glass. In the manufacturing method, at the time of connecting the starting rod and the dummy glass, a shape is adjusted in such a manner that an iron is brought into contact with a connection portion and is moved from a starting rod side toward a dummy glass side with appliance of a load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.17/848,367, filed on Jun. 23, 2022, which claims priority to JapanesePatent Application No. 2021-109906 filed in JP on Jul. 1, 2021, thecontents of which is incorporated herein by reference in their entirety.

BACKGROUND 1. Technical Field

The present invention relates to an optical fiber glass preform and amethod for manufacturing the optical fiber glass preform.

2. Related Art

As a method for manufacturing an optical fiber glass preform, a methodof sintering a porous glass preform obtained by a VAD method or an OVDmethod is known.

Patent Document 1 describes a method in which an optical fiber glasspreform is produced by depositing glass fine particles for a clad on anouter periphery of a starting member obtained by welding dummy glassesto both ends of a starting rod, that is, on an outer periphery of thestarting rod, joints between the starting rod and the dummy glasses, anda part of the dummy glasses, and heating a deposited porous glass in ahigh-temperature furnace to form a transparent clad glass.

Regarding a connection method between the starting rod and the dummyglass, Patent Document 2 describes a method of pressing an iron againsta connection portion and “reciprocating” the iron to smooth theconnection portion. In addition, Patent Document 3 describes a method inwhich the outer diameter of a bump-shaped portion is made equal to theouter diameter of the dummy glass by repeatedly pressing and separatingthe dummy glass and the starting rod while heating the connectionportion.

-   Patent Document 1: Japanese Patent Application Publication No.    H11-189428-   Patent Document 2: Japanese Patent Application Publication No.    H6-199533-   Patent Document 3: Japanese Patent Application Publication No.    2014-80299

However, when the optical fiber glass preform produced by theabove-described method was heated in a subsequent process in a statewhere a dummy glass 1 was bonded to a suspension shaft or the like andhanged down (FIG. 1A), and a load was applied, a separation 5 was likelyto occur in the vicinity of the joint (FIG. 1B). A damage 6 of theoptical fiber glass preform may occur due to the separation, which isproblematic (FIG. 1C).

In the starting rod, in addition to germanium doped to increase therefractive index of a core, an appropriate amount of fluorine may bedoped in a clad to form a depressed portion or trench portion with areduced refractive index. In particular, when the fluorine was doped inthe starting rod portion joined to a dummy rod, a separation was likelyto occur in the vicinity of the joint.

In this regard, the present invention has been made in view of theabove, and an object thereof is to suppress separation of the startingrod and the dummy glass.

GENERAL DISCLOSURE

In order to solve the above problems, an optical fiber glass preform ofthe present invention includes: a starting rod; a dummy glass configuredto be fitted into one end of the starting rod and integrally joined; anda clad glass configured to surround a part of the dummy glass and thestarting rod.

According to the present invention, the starting rod is fitted into oneend of the dummy glass and integrally joined, and thus even when thedummy glass is hanged down and heated in a state where a load isapplied, a separation in the vicinity of a joint is less likely tooccur.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view illustrating an example of a conventionalstarting rod, dummy glass connection, and optical fiber glass preform.

FIG. 1B is a schematic view illustrating the example of the conventionalstarting rod, dummy glass connection, and optical fiber glass preform.

FIG. 1C is a schematic view illustrating the example of the conventionalstarting rod, dummy glass connection, and optical fiber glass preform.

FIG. 2A is a schematic view illustrating an example of a starting rod, adummy glass connection, and an optical fiber glass preform according tothe present invention.

FIG. 2B is a schematic view illustrating the example of the startingrod, the dummy glass connection, and the optical fiber glass preformaccording to the present invention.

FIG. 2C is a schematic view illustrating the example of the startingrod, the dummy glass connection, and the optical fiber glass preformaccording to the present invention.

FIG. 2D is a schematic view illustrating the example of the startingrod, the dummy glass connection, and the optical fiber glass preformaccording to the present invention.

FIG. 2E is a schematic view illustrating the example of the startingrod, the dummy glass connection, and the optical fiber glass preformaccording to the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, an example of a method for manufacturing an optical fiberglass preform according to the present invention will be described withreference to the accompanying drawings. Note that the present inventionis not limited by the embodiment described below.

An optical fiber glass preform of the present embodiment is manufacturedby joining a dummy glass 1 and a starting rod 2 (FIGS. 2A to 2D) andforming a clad glass 4 to surround a part of the dummy glass 1 and thestarting rod 2 (FIG. 2E).

In the optical fiber glass preform of the present invention, the dummyglass 1 is fitted into the starting rod 2 and integrally joined at ajoint 3 between the dummy glass 1 and the starting rod 2. That is, apartof the dummy glass 1 is joined in the state of entering the starting rod2 side from the end surface of the starting rod 2. In a subsequentprocess, by supporting the dummy glass 1 joined in this manner andsuspending the optical fiber glass preform, a separation in the vicinityof the joint 3 is less likely to occur.

Such joining with the dummy glass 1 may be performed at both ends of thestarting rod 2, and in this case, there is an advantage that aseparation in the vicinity of the joint is less likely to occur evenwhen the dummy glasses 1 are supported and suspended at both the ends.

The dummy glass 1 and the starting rod 2 are preferably airtightlyjoined at the joint 3. When air bubbles remain in the joint 3 and areconfined, a separation between the starting rod 2 and the dummy glass 1may occur starting from the air bubbles.

The starting rod 2 is a member to be a central portion of the opticalfiber glass preform to be manufactured and is preferably added with adopant to correspond to the refractive index distribution of an intendedoptical fiber. For example, a hollow round rod shape is formed in whicha center is silica glass doped with germanium (Ge), and the surroundingarea is surrounded by pure silica glass or silica glass doped withfluorine (F). (The starting rod may have a hollow round rod shape.)

The shape of the joint 3 at which the joining end of the dummy glass 1is fitted into the joining end of the starting rod 2 is not particularlylimited, but it is preferable that the dummy glass 1 and the startingrod 2 are joined in a state where the central axes of thecross-sectional circles of the dummy glass 1 and the starting rod 2coincide with each other. In this way, a stress distribution in thecircumferential direction of the cross section becomes uniform at thejoint 3. When the dummy glass 1 is a solid round rod, a state ispreferable in which the central portion of the joint end of the dummyglass 1 is fitted into the joining end of the starting rod 2 (FIG. 2D).In particular, the present invention can be suitably applied to a casewhere the outer diameters of the starting rod and the dummy glass arethe same.

However, when the dummy glass containing a large amount of dopantimpurities is used, the viscosity of the dummy glass may be lower thanthat of the starting rod when heated to the same temperature. In thiscase, the joint may be processed such that the central portion of thejoining end of the starting rod is fitted into the joining end of thedummy glass. The effects similar to those of the present embodiment canalso be obtained by such processing.

As illustrated in FIGS. 2A to 2E, when the outer diameter of the dummyglass 1 is smaller than the outer diameter of the starting rod 2, it isalso preferable to form a state where the joining end of the dummy glass1 is entirely fitted into the joining end of the starting rod. On theother hand, when the outer diameter of the dummy glass is larger thanthe outer diameter of the starting rod, a state may be formed in whichthe joining end of the starting rod is entirely fitted into the joiningend of the dummy glass. The dummy glass may have a circular tube shape(hollow round rod), which reduces the weight of the dummy glass.

The present invention is particularly effective when the dummy glass 1,the starting rod 2, and the clad glass 4 contain glasses having dopantcompositions different from each other. In this case, the joint 3 of aconnection portion can be visually observed.

For example, the starting rod for an optical fiber is synthetic quartzglass to which dopants such as germanium (Ge) and fluorine (F) are addedto adjust the refractive index, while the clad glass is synthetic quartzglass hardly containing these dopants. In such a synthetic quartz glass,in order to reduce light absorption, OH groups are removed to have an OHgroup content of 1 ppm or less, and chlorine (Cl) of 100 ppm or more iscontained. On the other hand, inexpensive natural quartz glass is oftenused as the dummy glass.

When a set of three different types of glasses of dummy glass, startingrod, and clad glass is used for formation, stress tends to concentrateat the vicinity of the joint. In a subsequent process, thermal energy isapplied to the joint, and further, almost all the load of an opticalfiber glass preform body is applied, so that the separation of the jointeasily proceeds. The optical fiber glass preform of the presentinvention is effective in suppressing the separation of the joints ofsuch different glasses and is more effective particularly when thestarting rod is doped with fluorine. In this case, it is effective whenthe doping amount of fluorine is 0.7 wt % or less, and it is furthereffective when the doping amount of fluorine is 0.5 wt % or less.

In a method for manufacturing the optical fiber glass preform of thepresent invention, the distal ends of the dummy glass 1 and the startingrod 1 are faced to each other by a processing device such as a glasslathe (FIG. 2A) and then pressed and welded after being heated andsoftened by flame or the like (not illustrated), so that alarge-diameter portion 7 having an outer diameter larger than that ofthe dummy glass 1 or the starting rod 2 is produced at a joining portion(FIG. 2B). When a processing iron 8 is brought into contact with thelarge-diameter portion 7 to level the large-diameter portion, theprocessing iron 8 is brought into contact with the large-diameterportion and moved from the starting rod 2 side toward the dummy glass 1side while applying a load, thereby leveling the bulge of thelarge-diameter portion 7 (FIG. 2C) and forming the joint 3 in which thejoining end of the dummy glass 1 is fitted into the joining end of thestarting rod 2 (FIG. 2D). Here, “leveling” the bulge of thelarge-diameter portion 7 means equalizing the outer diameter of thelarge-diameter portion 7 so that a difference with the outer diameter ofthe starting rod 2 becomes small, it is preferable that the differencebetween both outer diameters is 10% or less of the outer diameter of thestarting rod 2, and it is more preferable that the difference is 5% orless. When the large-diameter portion 7 is not leveled, the flow ofglass fine particles blown to the starting rod 2 in a glass fineparticle deposition process as a subsequent process is impaired, and theformation of a glass fine particle deposition layer is likely to bedisturbed.

By adopting this method, the large-diameter portion 7 can be easilyshaped, and the processing of the dummy glass 1 and the starting rod 2can be completed. When the distal end of the dummy glass 1 is fittedinto the starting rod 2 (FIG. 2D), local stress and a load concentrationhardly occur, so that the separation of glass is less likely to occur.Note that the joint 3 of the connection portion indicated by a dottedline in the drawing can be visually observed.

The fitting depth of the dummy glass into the starting rod can beadjusted by, for example, the size of the diameter of the large-diameterportion formed by welding the end surfaces of the softened dummy glassand the starting rod to each other. When the large-diameter portion islarge, the fitting depth can be made large, and when the large-diameterportion is small, the fitting depth can be made small. In addition, theadjustment may be made by adjusting a heating temperature at the time ofmolding the large-diameter portion to change the degree of softening ofglass or by adjusting the load applied by the iron and the moving speedof the iron at the time of molding the large-diameter portion.

In the present invention, at least one of the end surfaces of thestarting rod and the dummy glass before being welded desirably has aconvex shape. When both end surfaces of the starting rod and the dummyglass are flat, air bubbles may remain at the time of welding both endsurfaces to be confined in the joint. When at least one of both endsurfaces is formed into a convex shape, the convex portion is weldedwhile being deformed to form the joint, so that air bubbles do notremain. In addition, when the large-diameter portion produced at thetime of welding is shaped with the processing iron, the large-diameterportion formed by connecting convex portions or a convex portion and aflat surface is smooth as compared with the large-diameter portionformed by connecting flat surfaces, and thus there is also an advantagethat it is easy to shape.

As for processing of a tip into a convex shape, it is preferable togrind the tip with a grinder or the like in advance. In the protrusionof a convex portion, the center may be raised, but the convex portionmay be deviated outward from the center.

A protruding amount is preferably 0.5 times or more and 2 times or lessa radius. When the protruding amount is less than 0.5 times, air bubblesare likely to remain, and when the protruding amount exceeds 2 times,the convex portion is likely to be deformed during heating by flame tohang down.

In the present invention, the outer diameter of the dummy glass ispreferably smaller than the outer diameter of the starting rod. Sincethe main purpose of the dummy glass is to support the starting rod inthe processes of glass fine particle deposition and sintering, it is notnecessary to make the dummy glass thicker than necessary. In addition,as described above, the joining end of the dummy glass can be easilyfitted into the joining end of the starting rod by applying the iron tothe starting rod side of the large-diameter portion (the portion wherethe outer diameter is increased) formed at the joint between thestarting rod and the dummy glass to apply a load toward the dummy glassside and leveling the large-diameter portion. (Although the size of thestarting rod is determined by the size of the intended optical fiberglass preform or the design of the refractive index distribution,) whenthe dummy glass is as thin as possible, the amount of heat, time, andload required for heating and joining can be reduced to facilitateprocessing. EXAMPLES

The starting rod manufactured by using a VAD method and made ofsynthetic quartz glass including a core was prepared to have an outerdiameter of 50 mm and a length of 1500 mm and have flat surfaces at bothends. The starting rod is distinguished into a manufacturing start sideand a manufacturing end side of the VAD method. In addition, thestarting rod prepared here is not doped with fluorine.

An end surface of the starting rod on the manufacturing start side ofthe VAD method was gripped by one chuck of the glass lathe towardanother chuck, a dummy glass made of natural quartz glass and having anouter diameter of 50 mm and a length of 500 mm was gripped by theanother chuck, and while rotating the chucks, the end surface of thestarting rod on the manufacturing start side and the end surface of thedummy glass were heated to about 2000° C. with oxyhydrogen flame emittedfrom a burner to be softened. This state is referred to as (a).

Example 1

From (a), the end surfaces were welded to each other by bringing thechucks close to each other, and the connection portion was bulged (alarge-diameter portion was formed). The maximum diameter of the bulgedconnection portion (large-diameter portion) was 54 mm. While continuingthe heating of the connection portion by the oxyhydrogen flame, anoperation of leveling the bulge (large-diameter portion) of theconnection portion was performed by bringing the processing iron made ofhigh purity carbon into contact with the starting rod side of the bulgedconnection portion (large-diameter portion) and moving the processingiron toward the dummy glass side while applying a load. The maximumdiameter of the connection portion after the operation was 51 mm. At thejoint, the dummy glass was fitted by 1 mm into the starting rod.

Example 2

From (a), the end surfaces were welded to each other by bringing thechucks close to each other, and the connection portion was bulged (alarge-diameter portion was formed). The maximum diameter of the bulgedconnection portion (large-diameter portion) was 56 mm. While continuingthe heating of the connection portion by the oxyhydrogen flame, anoperation of leveling the bulge (large-diameter portion) of theconnection portion was performed by bringing the processing iron made ofhigh purity carbon into contact with the starting rod side of the bulgedconnection portion (large-diameter portion) and moving the processingiron toward the dummy glass side while applying a load. The maximumdiameter of the connection portion after the operation was 52 mm. At thejoint, the dummy glass was fitted by 3 mm into the starting rod.

Example 3

From (a), the end surfaces were welded to each other by bringing thechucks close to each other, and the connection portion was bulged (alarge-diameter portion was formed). The maximum diameter of the bulgedconnection portion (large-diameter portion) was 57 mm. While continuingthe heating of the connection portion by the oxyhydrogen flame, anoperation of leveling the bulge (large-diameter portion) of theconnection portion was performed by bringing the processing iron made ofhigh purity carbon into contact with the starting rod side of the bulgedconnection portion (large-diameter portion) and moving the processingiron toward the dummy glass side while applying a load. The maximumdiameter of the connection portion after the operation was 52 mm. At thejoint, the dummy glass was fitted by 5 mm into the starting rod.

With respect to the connection body of the starting rod and the dummyglass of Examples 1, 2, and 3, the grip of the starting rod by the chuckwas released while the chuck for the dummy glass was gripped, anotherdummy glass made of natural quartz glass and having an outer diameter of50 mm and a length of 500 mm was gripped by the chuck on the releasedside, while the chucks are rotated, the end surfaces of the starting rodand the dummy glass were heated to about 2000° C. with oxyhydrogen flameemitted from the burner to be softened, and then the chucks were broughtclose to each other, so that the end surfaces of the dummy glass and thestarting rod on the manufacturing end side of the VAD method were weldedto form a large-diameter portion with a bulged connection portion. Themaximum diameter of the bulged connection portion (large-diameterportion) was 54 mm. While continuing the heating of the connectionportion (large-diameter portion) by the oxyhydrogen flame, an operationof leveling the bulge (large-diameter portion) of the connection portionwas performed by putting the processing iron on the bulged connectionportion (large-diameter portion) and reciprocating the processing ironbetween the starting rod side and the dummy glass side of the joint.Accordingly, a target having dummy glasses at both ends of the startingrod were formed.

The dummy glasses at both ends in the target formed in Examples 1, 2,and 3 were gripped by the chucks disposed at both ends in a chamber, andglass fine particles were deposited on the outer periphery of the targetby the OVD method to generate a porous glass preform having an outerdiameter of 300 mm. The dummy glass, which is joined to the starting rodon the manufacturing start side of the VAD method, of the porous glasspreform was connected to the tip of the shaft of the lifting mechanismof a dehydration sintering apparatus, the porous glass preform isinserted into a furnace core tube of the dehydration sintering apparatusin the state of being vertically suspended with the manufacturing startside of the VAD method in the starting rod directed upward and is moveddownward while being heated at 1500° C., and the porous glass issintered to be transparently vitrified, thereby manufacturing an opticalfiber glass preform.

In each of the configurations of Examples 1, 2, and 3, ten optical fiberglass preforms were manufactured, a load of 50 kgf was applied downwardin a state where the dummy glass on the VAD manufacturing start side wasgripped and suspended vertically, and the joint between the starting rodand the dummy glass on the VAD manufacturing start side was visuallyobserved to examine the occurrence rate of separation. The results areshown in Table 1.

TABLE 1 EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 FITTING DEPTH 1 3 5 (mm) FITTINGDEPTH/OUTER 2 6 10 DIAMETER OF STARTING ROD (%) SEPARATION 30 20 0OCCURRENCE RATE OF JOINT (%)

In all of Examples 1, 2, and 3, a high separation suppression effect ofa separation occurrence rate of 30% or less was confirmed. From thecomparison of Examples 1, 2, and 3, there is a tendency that the effectof suppressing the occurrence of separation is enhanced when the fittingdepth increases. Since the fitting depth of the dummy glass of Example 1into the starting rod was 1 mm which is the length corresponding to 2%of the outer diameter 50 mm of the starting rod, it is considered thatan excellent effect is exhibited particularly when the fitting depth ofthe dummy glass into the starting rod is equal to or larger than 2% ofthe outer diameter of the starting rod.

[Examples 4 to 7] As a separate study, the starting rod manufactured byusing the VAD method and made of synthetic quartz glass including a corewas prepared to have an outer diameter of 50 mm and a length of 1500 mm.In order to form a depressed portion on the outer periphery of the core,this starting rod was doped with fluorine which was divided into fivetypes of 0 wt % ( . . . Example 1), 0.1 wt % ( . . . Example 4), 0.3 wt% ( . . . Example 5), 0.5 wt % ( . . . Example 6), and 0.7 t % ( . . .Example 7). The fluorine concentration of the starting rod is anumerical value obtained by dissolving a part of a glass piece obtainedby pulverizing the starting rod and analyzing the result by ionchromatography. For each of these starting rods, ten optical fiber glasspreforms were manufactured in a procedure similar to in Example 1.

Comparative Examples 1 and 2

The starting rod manufactured by using the VAD method and made ofsynthetic quartz glass including a core was prepared to have an outerdiameter of 50 mm and a length of 1500 mm. In order to form a depressedtype on the outer periphery of the core, this starting rod was dopedwith fluorine which was divided into two types of 0.5 wt % ( . . .Comparative Example 1) and 0.7 wt % ( . . . Comparative Example 2),thereby reducing transmission loss of the optical fiber obtained bydrawing. The fluorine concentration of the starting rod is a numericalvalue obtained by dissolving a part of a glass piece obtained bypulverizing the starting rod and analyzing the result by ionchromatography.

For the starting rods of Comparative Examples 1 and 2, an end surface ofthe starting rod on the manufacturing start side of the VAD method wasgripped by one chuck of the glass lathe toward another chuck, a dummyglass made of natural quartz glass and having an outer diameter of 50 mmand a length of 500 mm was gripped by the another chuck, and whilerotating the chucks, the end surface of the starting rod on themanufacturing start side and the end surface of the dummy glass weresoftened by being heated to about 2000° C. with oxyhydrogen flameemitted from a burner. On the other hand, the end surfaces were weldedto each other by bringing the chucks close to each other, and theconnection portion was bulged (a large-diameter portion was formed). Themaximum diameter of the bulged connection portion (large-diameterportion) was 54 mm. Thereafter, while continuing the heating by theoxyhydrogen flame, an operation of leveling the bulge of the connectionportion was performed by putting the processing iron on the bulgedconnection portion (large-diameter portion) and reciprocating theprocessing iron between the starting rod side and the dummy glass sideof the joint. The maximum diameter of the connection portion after theoperation was 53 mm. However, unlike Examples 1 to 7, the joint betweenthe joining end of the dummy glass and the joining end of the startingrod was planarly joined.

With respect to the connection body of the starting rod and the dummyglass of Comparative Examples 1 and 2, the grip of the starting rod bythe chuck was released while the chuck for the dummy glass was gripped,another dummy glass made of natural quartz glass and having an outerdiameter of 50 mm and a length of 500 mm was gripped by the chuck on thereleased side, while rotating the chucks, the end surfaces of thestarting rod and the dummy glass were heated to about 2000° C. withoxyhydrogen flame emitted from the burner to be softened, and then thechucks were brought close to each other, so that the end surfaces of thedummy glass and the starting rod on the manufacturing end side of theVAD method were welded to form a large-diameter portion with a bulgedconnection portion. The maximum diameter of the bulged connectionportion (large-diameter portion) was 54 mm. While continuing the heatingof the connection portion (large-diameter portion) by the oxyhydrogenflame, an operation of leveling the bulge (large-diameter portion) ofthe connection portion was performed by putting the processing iron onthe bulged connection portion (large-diameter portion) and reciprocatingthe processing iron between the starting rod side and the dummy glassside of the joint. Accordingly, a target having dummy glasses at bothends of the starting rod were formed.

The dummy glasses at both ends in the target formed in ComparativeExamples 1 and 2 were gripped by the chucks disposed at both ends in achamber, and glass fine particles were deposited on the outer peripheryof the target by the OVD method to generate a porous glass preformhaving an outer diameter of 300 mm. The dummy glass, which is joined tothe starting rod on the manufacturing start side of the VAD method, ofthe porous glass preform was connected to the tip of the shaft of thelifting mechanism of the dehydration sintering apparatus, the porousglass preform is inserted into the furnace core tube of the dehydrationsintering apparatus in the state of being vertically suspended with themanufacturing start side of the VAD method in the starting rod directedupward and is moved downward while being heated at 1500° C., and theporous glass is sintered to be transparently vitrified, therebymanufacturing ten optical fiber glass preforms.

With respect to the above-described optical fiber glass preform, thejoint between the starting rod and the dummy glass on the manufacturingstart side was visually observed to examine the occurrence rate ofseparation. The results are shown in Table 2.

TABLE 2 COMPARATIVE COMPARATIVE EXAMPLE 1 EXAMPLE 4 EXAMPLE 5 EXAMPLE 6EXAMPLE 7 EXAMPLE 1 EXAMPLE 2 DOPING 0 0.1 0.3 0.5 0.7 0.5 0.7CONCENTRATION OF FLUORINE (wt %) FITTING DEPTH/ 2 2 2 2 2 0 (NO 0 (NOOUTER FITTING) FITTING) DIAMETER OF STARTING ROD (%) SEPARATION 30 30 4040 60 90 90 OCCURRENCE RATE OF JOINT (%)

From the comparison of Example 5 with Comparative Example 1 and Example7 with Comparative Example 2, it has been confirmed that even when thefluorine doping concentration of the starting rod is the same, theoccurrence of separation of the joint can be significantly suppressed bysetting the fitting depth/the outer diameter of the starting rod to 2%or more. The effect of the present invention could be confirmed in arange where the fluorine doping amount of the starting rod was 0.7 wt %or less. A particularly high effect could be confirmed when the fluorinedoping concentration of the starting rod was in a range of 0.5 wt % orless.

Example 8

As a separate study, the starting rod manufactured by using the VADmethod and made of synthetic quartz glass including a core was preparedto have an outer diameter of 50 mm and a length of 1500 mm. The tip ofthe starting rod on the manufacturing start side was ground into aconvex conical shape having a height of 30 mm by using a disc grinder.

Example 9

Both ends of the starting rod manufactured by using the VAD method, madeof synthetic quartz glass including a core, and having an outer diameterof 50 mm and a length of 1800 mm were gripped by the chucks on bothsides of the glass lathe, and while a position of 300 mm from themanufacturing start side is heated with the oxyhydrogen flame to besoftened, one chuck was moved to perform melting, so that the tip of thestarting rod having a length of 1500 mm on the manufacturing start sidewas molded into a convex parabolic taper shape having a height of 30 mm.

For the starting rods of Examples 8 and 9, ten optical fiber glasspreforms were manufactured in a procedure similar to in Example 1, andthe joint between the starting rod and the dummy glass on themanufacturing start side was visually observed to examine the occurrencerate of separation. The results are shown in Table 3.

TABLE 3 EXAMPLE EXAMPLE EXAMPLE 1 8 9 END SURFACE SHAPE FLAT CONEPARABOLIC OF STARTING ROD TAPER SEPARATION 30% 0% 0% OCCURRENCE RATE OFJOINT

In both Examples 8 and 9, the separation occurrence rate of the jointwas 0%. It is considered that since the tip became convex, bubbles didnot enter the joint, and thus, the starting point of the separationdisappeared, which affected the result. In Examples 8 and 9, the endsurface shape of the starting rod is changed, but the same effect can beobtained even when the end surface shape of the dummy glass is changed.

Example 10

As a separate study, the starting rod manufactured by using the VADmethod and made of synthetic quartz glass including a core was preparedto have an outer diameter of 50 mm and a length of 1500 mm. An endsurface of the starting rod on the manufacturing start side was graspedby one chuck of the glass lathe toward another chuck, a dummy glass madeof natural quartz glass and having an outer diameter of 40 mm and alength of 500 mm was gripped by the another chuck, while rotating thechucks, the end surface of the starting rod on the manufacturing startside and the end surface of the dummy glass were softened by beingheated to about 2000° C. with oxyhydrogen flame emitted from the burner,and then the chucks were brought close to each other to weld the endsurfaces to each other to bulge the connection portion. The maximumdiameter of the bulged connection portion was 48 mm. On the other hand,while continuing the heating of the connection portion by theoxyhydrogen flame, an operation of leveling the bulge of the connectionportion was performed by bringing the processing iron made of highpurity carbon into contact from the starting rod side of the bulgedconnection portion toward the dummy glass side. The maximum diameter ofthe connection portion after the operation was 45 mm.

Example 11

An end surface of the starting rod on the manufacturing start side wasgrasped by one chuck of the glass lathe toward another chuck, a dummyglass made of natural quartz glass and having an outer diameter of 60 mmand a length of 500 mm was gripped by the another chuck, while rotatingthe chucks, the end surface of the starting rod on the manufacturingstart side and the end surface of the dummy glass were softened by beingheated to about 2000° C. with oxyhydrogen flame emitted from the burner,and then the chucks were brought close to each other to weld the endsurfaces to each other to bulge the connection portion. The maximumdiameter of the bulged connection portion was 59 mm. On the other hand,while continuing the heating of the connection portion by theoxyhydrogen flame, an operation of leveling the bulge of the connectionportion was performed by bringing the processing iron made of highpurity carbon into contact from the starting rod side of the bulgedconnection portion toward the dummy glass side. The maximum diameter ofthe connection portion after the operation was 54 mm.

For the joining body of the starting rod and the dummy glass of Examples10 and 11, ten optical fiber glass preforms were manufactured in aprocedure similar to in Example 1, and the joint between the startingrod and the dummy glass on the manufacturing start side was visuallyobserved to examine the occurrence rate of separation. The results areshown in Table 4.

TABLE 4 EXAMPLE EXAMPLE EXAMPLE 1 10 11 OUTER DIAMETER OF 50 50 50STARTING ROD (mm) OUTER DIAMETER OF 50 40 60 DUMMY GLASS (mm) SEPARATIONOCCURRENCE 30%  0% 10% RATE OF JOINT (%)

In Example 10 in which the outer diameter of the starting rod was largerthan the outer diameter of the dummy glass, the separation occurrencerate of the joint was 0%. In consideration of the cost of the dummyglass, it is considered desirable that the outer diameter of thestarting rod is larger than the outer diameter of the dummy glass.

The present invention is not limited to the above-described embodiment,and proper modifications and improvements can be made arbitrarily.

What is claimed is:
 1. A method for manufacturing an optical fiber glasspreform, the method comprising: heating and joining one end of astarting rod and one end of a dummy glass facing each other; forming alarge-diameter portion having an outer diameter larger than those of thedummy glass and the starting rod at a joining portion between thestarting rod and the dummy glass; bringing an iron into contact with thelarge-diameter portion and moving the iron from the starting rod side ofthe large-diameter portion toward the dummy glass side while applying aload to form a joint where a joining end of the dummy glass is fittedinto a joining end of the starting rod; depositing a glass fine particlelayer on an outer periphery of the dummy glass and the starting rod; andheating the glass fine particle layer to be transparent.
 2. The methodfor manufacturing an optical fiber glass preform according to claim 1,wherein the starting rod is doped with fluorine.
 3. The method formanufacturing an optical fiber glass preform according to claim 1,wherein the starting rod is doped with fluorine of 0.7 wt % or less. 4.The method for manufacturing an optical fiber glass preform according toclaim 1, wherein the starting rod is doped with fluorine of 0.5 wt % orless.
 5. The method for manufacturing an optical fiber glass preformaccording to claim 1, further comprising: processing at least one of oneend of the starting rod or one end of the dummy glass into a convexshape.
 6. The method for manufacturing an optical fiber glass preformaccording to claim 1, wherein an outer diameter of the dummy glass isequal to or smaller than an outer diameter of the starting rod.